While browsing for an answer to a question I had, I found someone answered a question that was almost 3 years old. The post already has accepted answer. My question is: What's the use of answering old questions?

There are more than one ways to skin a cat. I don't know much about Android, but given that the answer has downvotes, I'm assuming its incorrect. However, if it were an valid alternate solution, there's a good chance that someone would find it later and find it useful.

I think it's been made pretty clear that SO isn't about the OP. It's more about the people coming to the question afterwards. Also, the OP will see the notification that an answer has been posted, and it could be helpful to them as well.

The poster of that answer explained in detail why he posted the answer:

[Stack Overflow] is a global knowledge base, which generally prevents you for asking new question if they already were covered. So for example if someone has the problem now, he/she still should use this question pile. So having an alternative answer is really valuable. So far no one marked as the answer post had value for me.

In other words, the accepted answer didn't work for him, and asking a new question would have caused a close as duplicate, so he posted what did work for him on the existing question.